Mark fast/selectors/nth-child-of-basics as Slow
https://bugs.webkit.org/show_bug.cgi?id=137149

Reviewed by Benjamin Poulain.

Since there are many tests in fast/selectors/nth-child-of-basics,
it takes long time and sometime LayoutTests timeout[1].

[1]: https://build.webkit.org/builders/GTK%20Linux%2064-bit%20Release%20(Tests)/builds/3042

* TestExpectations:
Mark fast/selectors/nth-child-of-basics as Slow.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@174032 268f45cc-cd09-0410-ab3c-d52691b4dbfc